Innovative Designs for the Suspension System of Horizontal-Axis Washing Machines: Secondary Suspensions and Tuned Mass Dampers

Cabinet vibrations during spinning cycles of washing machines are often perceived negatively by customers, both in acoustic and visual terms. Therefore, in a market that is more and more oriented towards customer satisfaction and appliance efficiency, reducing cabinet vibrations and noise is a very attractive target. Previous experimental campaigns have highlighted that currently installed dry-friction dampers have highly non-linear characteristics and they are the main source of broadband forcing on the cabinet panels. To overcome these negative effects, two innovative designs for the suspension system have been studied and are here presented: the first solution is a secondary suspension system, designed to filter out the high frequency force components introduced by the dampers and therefore to mitigate panel vibrations; the second solution is more radical and it consists in removing the existing dampers and to substitute them with a tuned mass damper (TMD), directly fixed to the oscillating group. These solutions are a compromise between cost and efficiency, since a linear oil damper would impact excessively on the final cost of the appliance. The secondary suspension system has been designed to meet the very strict requirements of the manufacturer: low cost (which implies small mass) and applicable to the existing machine without any modification of any part other than the dampers themselves. The desired results have been achieved increasing the mass of the damper rod and introducing a fixed-fixed (or hinged-hinged) beam spring. The prototype of the secondary suspension has been tested independently and assembled in the washing machine, and the vibratory and acoustic results will be reported. The introduction of a TMD and the contextual removal of the dry-friction dampers, on the contrary, requires the modification of the oscillating group in order to work efficiently. As a matter of fact, a principle for the design of oscillating groups is the space optimization, which usually leads to asymmetric distributions of mass of the oscillating group, that in their turn lead to asymmetrically coupled mode shapes. This dynamic characteristic does not suit the design of a simple and compact TMD, with a limited number of degrees of freedom. We show that careful mass distribution and a specifically designed 2-dofs TMD can efficiently substitute the original dampers. An elemental prototype of the so defined TMD has been tested on a shaking table, and it seems an effective alternative to classical suspension systems.